Superflares on Ordinary Solar-Type Stars

Short-duration flares are well known to occur on cool main-sequence stars as well as on many types of "exotic" stars. Ordinary main-sequence stars are usually pictured as being static on timescales of millions or billions of years. Our Sun has occasional flares involving up to ~10³¹ ergs that produce optical brightenings too small in amplitude to be detected in disk-integrated brightness. However, we identify nine cases of superflares involving 10³³-10³⁸ ergs on normal solar-type stars. That is, these stars are on or near the main sequence, are of spectral class F8-G8, are single (or in very wide binaries), are not rapid rotators, and are not exceedingly young in age. This class of stars includes many of those recently discovered to have planets as well as our own Sun, and the consequences for any life on surrounding planets could be profound. For the case of the Sun, historical records suggest that no superflares have occurred in the last two millennia.